Genetics Basics: quick revision, 13 bites

Genetics, Breeding, and Heredity

Genes pass traits; breeding controls them.

**Genetics** is the branch of biology that studies genes, heredity, and variation in living organisms. **Breeding** is the controlled reproduction of organisms (here, fish) to produce offspring with desired traits. **Heredity** is the passing of traits from parents to offspring through genes.

Takeaway. Genetics explains inheritance; breeding applies it to produce desired traits.

Variation and Trait

Same species, different features.

**Variation** refers to differences in characteristics among individuals of the same species, arising from genetic and/or environmental causes. A **trait** (or character) is an observable feature or characteristic of an organism.

Takeaway. Variation is the raw material for selection; traits are the features we observe.

Phenotype vs Genotype

What you see vs what genes say.

Takeaway. Phenotype = genotype + environment; genotype is the underlying genetic makeup.

Genome

The complete genetic instruction set.

The **genome** is the complete set of genetic material (DNA) present in an organism. It includes all genes and non-coding regions.

Takeaway. Genome = entire DNA content of an organism.

Gene and Chromosome

Units of heredity and their carriers.

A **gene** is the basic unit of heredity; a segment of DNA that codes for a specific protein or trait and occupies a specific location (**locus**) on a chromosome. A **chromosome** is a thread-like structure of DNA and associated proteins (**histones**) found in the nucleus, carrying genes in linear order.

Takeaway. Genes are DNA segments; chromosomes are the organized structures that carry them.

DNA and RNA

Two nucleic acids with distinct roles.

**DNA** (Deoxyribonucleic Acid) is the molecule that carries genetic information, composed of two strands forming a double helix, built from nucleotides (A, T, G, C). **RNA** (Ribonucleic Acid) is a nucleic acid involved in decoding and expressing genetic information (mRNA, tRNA, rRNA).

Takeaway. DNA stores genetic information; RNA helps express it.

Nucleotide

Building block of DNA and RNA.

A **nucleotide** is the building block of DNA/RNA, consisting of a sugar, a phosphate group, and a nitrogenous base.

Takeaway. Nucleotides are the monomers that make up nucleic acids.

Locus and Allele

Location and variant forms of a gene.

**Locus** (plural: loci) is the specific physical location of a gene on a chromosome. An **allele** is one of two or more alternative forms of a gene occupying the same locus.

Takeaway. Locus is the address; alleles are the different versions of a gene at that address.

Homologous Chromosomes

Matching chromosomes from each parent.

**Homologous chromosomes** are a pair of chromosomes (one from each parent) that are similar in size, shape, and gene content.

Takeaway. Homologous chromosomes carry the same genes but may have different alleles.

Diploid vs Haploid

Two sets vs one set of chromosomes.

Takeaway. Body cells are diploid (2n); gametes are haploid (n).

Karyotype

Chromosome portrait of a species.

A **karyotype** is the number, size, and shape of chromosomes characteristic of a species, usually displayed as an ordered image.

Takeaway. Karyotype reveals the chromosomal makeup of a species.

Chromatid and Centromere

Parts of a duplicated chromosome.

A **chromatid** is one of the two identical strands of a duplicated chromosome, joined at the **centromere**. The centromere is the constricted region that holds sister chromatids together and attaches to spindle fibers during cell division.

Takeaway. Sister chromatids are joined at the centromere until cell division separates them.

Mitosis vs Meiosis

Growth vs gamete production.

Takeaway. Mitosis makes identical diploid cells; meiosis makes diverse haploid gametes.